1. Field of the Invention
The present invention relates to a liquid crystal display device and fabrication method thereof, and more particularly to a seal pattern of a liquid crystal display device and fabrication method thereof.
2. Discussion of the Related Art
A liquid crystal display (LCD) device is an apparatus that displays images according to a change of a transmittance. Two substrates are disposed having electrodes in such a way that the surfaces of the electrodes face each other. A liquid crystal layer is interposed therebetween and the liquid crystal molecules are aligned by an electric field generated from a voltage between the electrodes. The LCD device is fabricated by the processes of forming a lower substrate, referred to as an array substrate, having thin film transistors (TFTs) and pixel electrodes; forming an upper substrate, referred to as a color filter substrate, having common electrodes and color filters; forming a liquid crystal cell by aligning and attaching the substrates; injecting the liquid crystal materials; and sealing and attaching a polarization film.
In a conventional LCD device, since a plurality of liquid crystal cells are simultaneously formed on a wide area substrate, a process of cutting the substrate into the unit liquid crystal cells is needed after an assembly process.
FIG. 1 is a flow chart illustrating a fabrication process of a conventional liquid crystal cell for an LCD device.
At step ST1, the lower and upper substrates are formed that include TFTs and color filters, respectively. The lower substrate is formed by repeating deposition and patterning steps of a thin film and several masks. Recently, a fabrication process that reduces cost by decreasing the number of masks has been investigated. The upper substrate is formed by subsequently making a black matrix, red (R)/green (G)/blue (B) color filters and a common electrode. The black matrix distinguishes the color filters and prevents light leakage of a non-pixel area. The color filter can be formed by a dyeing method, a printing method, a pigment dispersion method or an electro-deposition method; the pigment dispersion method is most widely employed.
At step ST2, an orientation film that determines an initial orientation of the liquid crystal layer is formed on the upper and lower substrates. This step includes deposition and alignment of a polymeric thin film along a specific direction. An organic material of the polyimide series is mainly used as the orientation film and a rubbing method is mainly used as the aligning method of the orientation film, respectively. The rubbing method consists of rubbing the orientation film along the specific direction by a rubbing cloth, and has advantages such as easy orientation treatment, suitability to mass production, high stability of the orientation and easy controllability of a pre-tilt angle.
At step ST3, a seal pattern that forms a gap for liquid crystal material injection and prevents leakage of the liquid crystal material is formed on one substrate. The seal patterning process involves forming a desired pattern by application of a thermosetting plastic. A screen print method using a screen mask and a seal dispenser method using a dispenser are used for the seal patterning process. For the simplicity of fabrication, the screen print method has mainly been used. However, since the screen mask is not suitable for a wide substrate and a contamination by contact between the mask and the orientation film often occurs, use of the seal dispenser method has gradually increased.
At step ST4, a spacer having a specific size to maintain a precise and uniform gap between the upper and lower substrates is deposited by spraying the spacer onto one of the upper and lower substrates. The spacer spray method can be divided into two different types: a wet spray method that involves spraying a mixture of alcohol and spacer material and a dry spray method that involves spraying spacer material alone. Furthermore, the dry spray method can be sub-divided into two different types: an electrostatic spray method that uses electrostatic force, and a non-electric spray method that uses gas pressure. Since the liquid crystal cell structure is susceptible to damage from static electricity, the non-electric method is mainly used.
At step ST5, the upper and lower substrates are attached by pressure-resistant hardening of the seal pattern.
At step ST6, the attached liquid crystal substrate is divided into unit cells. A cell cutting process includes a scribe process that forms cutting lines on a surface of the substrate using a diamond pen, a hardness of which is higher than a hardness of the glass substrate, and a break process that divides the unit cells by force.
At step ST7, a liquid crystal material is injected into the unit cells. A vacuum injection method using pressure difference between the inside and outside of the unit cells is commonly used as an effective injection method. Since fine air bubbles included in the liquid crystal material can deteriorate the display property of the unit cells, a bubble-eliminating process, in which the cells are kept in a vacuum state for a long period of time, is required.
After finishing the liquid crystal material injection, an injection hole is sealed to prevent leakage of the liquid crystal material. Generally, a ultra violet (UV) curable resin is deposited onto the injection hole by use of a dispenser and then ultra violet light is irradiated on the resin, thereby hardening the resin and sealing the injection hole. Polarization films are attached on outer surfaces of the unit cell and a driving circuit is connected to the unit cell using an attachment process. After the attachment process, a substrate etching process, in which the outer surfaces of the upper and lower substrates are etched to reduce the thickness of the substrates, is performed according to the desired lightening of the substrate.
FIG. 2 is a flow chart illustrating the substrate etching process.
At step ST8, impurities made during the previous processes are eliminated. If there are impurities on the outer surfaces of the attached substrates, etching deterioration such as an under-etching in the vicinity of the impurities occurs and the surfaces of the substrates become rough. Accordingly, since a diffused reflection or a refraction of the light can occur, the impurities are eliminated with a cleaning solution such as isopropyl alcohol (IPA) or deionized water (DI).
At step ST9, the attached substrates are etched. Generally, a glass substrate is used for the liquid crystal substrate and about 60% of the substrate comprises silicon dioxide (SiO2). Therefore, the substrate can be etched with a solution of hydrofluoric (HF) acid, for example, a diluted solution with about 15% concentration, which is an etching solution for SiO2.
At step ST10 and ST11, a residue of the HF solution is removed and the substrates are dried.
However, since a seal pattern can be damaged during the substrate etching process, a method of preventing the damage by forming a dual seal pattern is suggested.
FIG. 3 is a plan view of a seal pattern for 6, 13.3-inch unit cells in the glass substrate having an area of 590×670 mm2.
As shown, 6 unit cells, “A, B, C, D, E and F” are formed on the substrate 10. Each unit cell has a main seal pattern 21 and an injection hole 22 is formed at the lower center of each main seal pattern 21. Furthermore, first, second and third sub-seal patterns 31, 32 and 33 are formed at the outer side of the main seal patterns 21 and surrounding the main seal pattern 21 of each unit cell. The sub-seal patterns 31, 32 and 33 not only have the function of protecting the main seal pattern 21 from the etching solution but also have the function of venting air between the upper and lower substrates during the attachment process. In the case of the open sub-seal patterns 31, 32 and 33 having an open portion to vent the air with ease, the permeated etching solution can cause damage to the main seal pattern 21, for example, a corrosion of pads or a substrate damage. In contrast, in the case of the closed sub-seal patterns 31, 32 and 33 without the open portion to protect the main seal pattern 21 from the etching solution, deterioration can occur, for example, light leakage or a low display quality owing to the lack of the liquid crystal material injection. Therefore, the sub-seal patterns 31, 32 and 33 are formed to have a structure to improve the above drawbacks. However, if a plurality of unit liquid crystal cells are formed on one substrate shown in FIG. 3, deterioration resulting from a low margin of the sub-seal patterns 31, 32 and 33 can occur so that a yield is only about 60% to 70%.